An Alarm Indication Signal (AIS) is a signal transmitted in lieu of a normal signal to maintain transmission continuity, and indicate to a receiving terminal that there is a fault located somewhere along the transmission path, which could be source node, intermediate nodes, or any links along the path. An AIS has been used by Transport networks to indicate upstream alarms. When downstream nodes receive alarms from their upstream nodes, they can suppress secondary alarms which are caused by the upstream faults.
AIS alarm propagation has been used in conventional transport networks for a long time. However, such approaches are not effective in a packet based network for propagating faults, especially in a connection-less oriented packet network, where each node can be connected to its peers via multiple connections. Such propagation used by circuit networks can flood a packet network if there are many upstream faults.
IEEE802.1ag/D4.1 has proposed another form of AIS for provider edge nodes. This form multicasts an AIS signal to an entire administration domain, so that bridges can suppress alarms of losing its connectivity to their peers. However, there are issues with the proposed methods. IEEE802.1ag/D4.1 provides two possible ways to send AIS to affected nodes. A first method is to let a provider edge node send periodic AIS message to all the nodes in an administration domain. This method can introduce too many messages, flooding an administrative domain. The excessive messages can cause congestion and unnecessary traffic within the administration domain. A second method only sends one AIS message when a provider edge detects failure. Subsequently, an AIS “clear” message is sent when the provider node recovers from connectivity failure. In this case, even though the number of AIS messages in the administrative domain may be reduced, the AIS “clear” message may not be sent to newly added bridges when the provider node recovers from its connectivity failure, or nodes being created after the failure occur may not get a failure message.
Therefore, there is a need for a system that effectively enables fault propagation and achieve alarm suppression, in a multipoint packet based network.